1. Field of the Invention
The present invention relates to X-ray detecting devices, and more particularly, to driving methods for improving image quality and to X-ray detecting apparatus driven by those methods.
2. Description of the Related Art
Some medical and scientific imaging systems produce an image by detecting visible light. However, most medical and scientific imaging systems detect either infrared or X-rays. For example, X-ray imaging systems used in medical applications provide images of the interior of a human body. Such systems have typically used an X-ray sensitive film. However, newer systems often use an X-ray detecting device that produces electrical signal in response to received X-rays.
Referring now to FIG. 1 and FIG. 2, a typical X-ray detecting device includes gate lines GLn-1 and GLn and data lines DL (only one shown) that cross the gate lines. Those lines are disposed on a glass substrate 20. A thin film transistor (TFT) is provided at each intersection of a gate line (GLn-1 and GLn) and a data line DL. A pixel electrode 15 is connected to the TFT, and a capacitor Cst is connected between the pixel electrode 15 and a ground. An upper electrode 12, having a dielectric layer 13, is connected to a high voltage generator 11. A photosensitive layer 14 is disposed between the dielectric layer 13 and the pixel electrode 15.
Still referring to those figures, a gate electrode 18 of each TFT is connected to a gate line (GLn-1 and GLn in FIG. 2), while a source electrode 17 of each TFT is connected to a data line DL. A drain electrode 16 of each TFT is connected to a pixel electrode 15. The TFTs selectively respond to scanning signals that are sequentially applied to the gate lines GLn-1 and GLn so as to apply a current in each TFT capacitor Cst to a data line DL.
The pixel electrode 15 is at a pixel area between gate lines GLn-1 and GLn and data lines DL. The pixel electrode 15 supplies electric charges produced within the photosensitive layer 14 to the capacitor Cst in response to the high voltage from the high voltage generator 11.
Still referring to FIGS. 1 and 2, X-rays irradiated through an object pass through the upper electrode 12 and the dielectric layer 13 and into the photosensitive layer 14. The photosensitive layer 14 converts those received X-rays into electron-hole pairs. The high voltage (of several kV) from the high voltage generator 11 that is applied (via the upper electrode 12 and the dielectric layer 13) across the photosensitive layer 14 separates the electrons and holes. The holes are collected by the pixel electrode 15 and are stored in the capacitor Cst.
When a scanning signal is applied to the gate electrode 18 of a TFT, a channel is formed between the source electrode 17 and the drain electrode 16 of that TFT. Electric charges stored in the capacitor Cst associated with that gate electrode 18 are then supplied, via the drain electrode 16 and the source electrode 17, to a data line DL.
Referring now to FIG. 3, a driving apparatus for driving an X-ray detecting device includes an X-ray detecting panel 20 having X-ray sensing cells PXL arranged in a matrix, a scan driver 21 for sequentially applying scanning signals to m gate lines (GL1 to GLm), and a data reader 22 for reading data on n data lines (DL1 to DLn). Each X-ray sensing cells PXL is substantially identical to the cell shown in FIG. 1 and FIG. 2. When scanning signals from the scan driver 21 are sequentially applied to the gate lines (GL1 to GLm), X-ray data is supplied, via the n data lines (DL1 to DLn), to the data reader 22.
As shown in FIG. 4, the data reader 22 includes charge amplifiers 231 to 23n, and samplers & holders 241 to 24n. The charge amplifiers are respectively connected to associated n data lines DL1 to DLn. A shift register 25 latches data from the samplers & holders 241 to 24n. Each charge amplifier 231 to 23n amplifies charge supplied on a data line (DL1 to DLn) by a current gain. The amplified currents are applied to the samplers and holders 241 to 24n. The samplers & holders 241 to 24n sample the data from the charge amplifiers 231 to 23n and apply sampled data to the shift register 25. The shift register 25 has n stages, corresponding to the n data lines DL1 to DLn, that sequentially latch the data from the samplers & holders 241 to 24n. Furthermore, the shift register 25 supplies latched data to an output circuit (which is not shown). The output circuit converts analog data from the data reader 22 into digital data that is applied to a display device (also not shown).
1X-ray detecting devices preferably have both a high resolution and a large viewing area. Recently, a scheme has been developed in which two or four X-ray detecting panels 20 are combined into a large-screen composite X-ray panel in such a way as to achieve both a high resolution and a large viewing area:. Such a composite X-ray panel has advantages over a comparable integral X-ray detecting panel in that scanning a composite X-ray panel can be performed by simultaneously scanning individual panels, which leads to a reduced scan time and to reduced signal attenuation caused by signal delay.
A typical composite X-ray panel will be described in conjunction with FIG. 5 and FIG. 6. Referring to those figures, such a composite X-ray panel is made by horizontally and vertically combining four individual X-ray detecting panels, #1 to #4.
A driving apparatus for driving this composite X-ray panel includes scan drivers 211 to 214 for respectively scanning the first to fourth X-ray detecting panels #1 to #4, and data readers 221 to 224 for reading data.
The first X-ray detecting panel #1 responds to scanning signals from the first scan driver 211 in a sequence that runs from the 1st scan line SC1, which is at the upper end, to the (M/2)th scan line SCM/2, which is near the vertical center. At the same time, data from the first X-ray detecting panel #1 is sequentially read from the 1st data line D1, at the left side, to the (N/2)th data line DN/2, which is near the horizontal center, by means of the first data driver 221.
The second X-ray detecting panel #2 responds to scanning signals from the second scan driver 212 in sequence from the 1st scan line SC1, at the upper end, to the (M/2)th scan line SCM/2 which is near the vertical center. At the same time, data from the second X-ray detecting panel #2 is sequentially read from a ((N/2)+1)th data line, near the horizontal center, to an Nth data line DN on the right side by means of the second data driver 222.
The third X-ray detecting panel #3 responds to scanning signals from the third scan driver 213 in a sequence that runs from the ((M/2)+1)th scan line near the vertical center to the Mth scan line SCM at the bottom. At the same time, data from the third X-ray detecting panel #3 is sequentially read from a 1st data line D1 at the left to a (N/2)th data line DN/2 near the horizontal center by means of the third data driver 223.
The fourth X-ray detecting panel #4 responds to scanning signals from the fourth scan driver 214 in a sequence that runs from the ((M/2)+1)th scan line near the vertical center to the Mth scan line SCM. At the same time, data from the fourth X-ray detecting panel #4 is sequentially read from an ((N/2)+1)th data to the Nth data DN by means of the fourth data driver 224.
Such a composite X-ray panel can reduce the time required for data reading to ¼ that of a single X-ray panel having the same resolution and dimensions because the gate lines and the data lines are divided at boundaries between adjacent X-ray detecting panels, and because the divided X-ray detecting panels can be scanned simultaneously. Furthermore, the composite X-ray panel can reduce both data reading delays caused by an RC time constant associated with the data lines and noise because the data line length is ½ that of a single X-ray panel having the same resolution and size.
However, the composite X-ray panel suffers from reduced image quality. In particular, the center area brightness is reduced. Unfortunately, the center area is both the most sensitive area to the human observer and typically contains the most important reading information. Center area brightness reduction is caused by the fact that as the delay between charging and scanning a capacitor Cst increases, so does charge leakage. For instance, the capacitors Cst associated with the 1st scan line SC1 and the ((M/2)+1)th scan line SCM/2+1, which are read first, have a relatively small charge leakage, whereas the capacitors Cst associated with the (M/2)th scan line SCM/2 and the Mth scan line SCM have a relatively large charge leakage because they are scanned last. As a result, referring now to FIG. 7, in a conventional composite X-ray panel the center area and the area at the bottom right edge, both indicated by dotted lines, have significantly more leakage, and thus have reduced brightness, than other areas.
Therefore, a method of reducing charge leakage and image brightness problems in composite X-ray panels would be beneficial.